The last part of the gallery shows the Parking Brake wire and the Ground wire.

You can use the wire tap that is already on the parking brake wire to tap into the ground wire shown. You could also use a tap to connect the Mute wire to the ground wire.

I have confirmed this and you only have to use that one wire to ground. I tapped into the ground on the wiring harness in case I ever need to change the other harness (if I get that new truck I'm eyeing).